Manufacturing method of semiconductor laser of patterned-substrate type

ABSTRACT

A semiconductor laser of a patterned-substrate type comprises the patterned-substrate having a sloped portion and a planar portion, and a plurality of semiconductor layers formed on the patterned-substrate including a heterostructure. By controlling condition for growing a specific semiconductor layer, a preferable ratio of a sloped portion thickness to a planar portion thickness of the semiconductor layer can be obtained, which enables a lasing current of the laser to be confined in a restricted region, and this results in obtaining a high efficiency and a high power output of the semiconductor laser.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a manufacturing method of semiconductorlasers and structure thereof. More particularly, this invention relatesto a manufacturing method of semiconductor lasers in whichpatterned-substrate is used.

Present-day semiconductor lasers are required to have features of loweroscillation threshold current, higher efficiency and higher poweroutput. The semiconductor lasers utilizing compound semiconductormaterial such as GaInP and AlGaInP group can emit visible lasing lightof 0.6 μm wavelength and are useful as a light source in POS (Point ofSales) terminals, optical disk apparatus, and laser printers.

2. Description of the Related Art

As the semiconductor lasers as described above, a semiconductor lasercomprising an AlGaInP-GaInP-AlGaInP double-heterostructure is wellknown.

A typical prior art semiconductor laser of an AlGaInP-GaInP-AlGaInPdouble-heterostructure type is disclosed in Japanese Unexamined PatentPublication disclosed in Japanese Unexamined Patent PublicationTokkai-SHO 62-200786. A cross sectional view of this type is shown inFIG. 1. On an n-GaAs substrate 21, double-heterostructure of ann-AlGaInP lower clad layer 22, a GaInP active layer 23 and a p-AlGaInPupper clad layer 24 which includes a mesa-stripe portion 24a are formedin this order. Current-blocking layers 26 of n-GaAs are formed on theupper clad layer 24 so as to bury the mesa-stripe 24a, and a p-GaInPcontact layer 25 and a p-GaAs contact layer 27 are formed on themesa-stripe 24a, and a p-GaAs contact layer 28 covers the entireunderlying structure.

In the above structure, the mesa-stripe 24a of p-AlGaInP has a functionof guiding the lasing light, however, the current-blocking layers 26 onboth sides thereof work as a loss-guide. The semiconductor laser of thistype, therefore, has a demerit that a high efficiency and high poweroutput can not be expected.

As another type of semiconductor lasers of the prior art, a buriedheterostructure type is known, in which a AlGaInP-GaInP-AlGaInP doubleheterostructure is etched until a stripe-shaped ridge is formed, and theremoved portions on both sides of the stripe-shaped ridge are buriedwith current-blocking layers. However, this type also comprisesdifficulties in fabrication steps.

To solve the above problems, a semiconductor laser structure of thepatterned-substrate type shown in FIG. 2 is proposed by Japanese PatentApplication Tokugan-Hei 3-92341 having priority date of Jun. 20, 1990.(The same patent was filed as U.S. patent application No. 691,620 datedApr. 25, 1991, and also filed as European Patent Application No.91-303783.4). In the application, an AlGaInP-GaInP-AlGaInP doubleheterostructure is formed on the patterned-substrate having a mesastripe 31a of a p-GaAs substrate 31 and n-GaAs current blocking layers32. On the patterned-substrate, a p-AlGaInP spike reduction layer 33, ap-AlGaInP lower clad layer 34, a GaInP active layer 35, an n-AlGaInPupper clad layer 36 and an n-GaAs contact layer 37 are formed in thisorder. Each of these stacked layers forming a heterostructure hasdownward inclination on both sides of the mesa-stripe 31a forming agradual slope and continues to a comparatively flat layer. The structuredoes not include a loss-guide structure, and control of transverselasing mode is attained by a bent shape of the heterostructure.

In conjunction with the laser structure of the above patterned-substratetype, Japanese Unexamined Patent Publication Tokkai-Hei 4-133315discloses a method of improving a shape of the patterned-substratehaving the GaAs substrate 31 with the mesa stripe 31a and thecurrent-blocking layers 32 on the substrate 31 shown in FIG. 2. A SiO₂layer is first deposited on a flat original GaAs substrate, and next theSiO₂ layer is patterned in a stripe shape and, thereafter the substrateis subjected to a wet-etching process using, for example, a mixedsolution of H₂ SO₄, H₂ O₂ and H₂ O. Next, the n-GaAs current blockinglayers 32 are deposited thereon by a MOVPE (Metal Organic Vapor PhaseEpitaxy) method.

The semiconductor lasers of the patterned-substrate type havecapabilities of high efficiency and high power output, however, acurrent confining function is not satisfactory because the verticallasing current through the stacked layers is apt to spread outwardly inthe transverse direction. Further, the heterostructure above themesa-stripe 31a (lasing portion) is not formed of ideal flat layers butit comprises small undulation which causes splitting of the transverselasing mode with the result of deteriorating a far field pattern of thelasing light.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a manufacturing methodand a structure of the semiconductor laser of the patterned-substratetype, in which a current confining characteristic is well controlled andlasing region is stable.

Another object of the present invention is to provide a semiconductorlaser of the patterned-substrate type having a low threshold current, ahigh efficiency and a high power output.

Still another object of the present invention is to provide amanufacturing method of a semiconductor laser of the patterned-substratetype, in which a grown clad layer above the patterned substrate has apredetermined thickness ratio, the ratio being defined for a slopedportion thickness to a planar portion thickness of the grown layer.

The above objects are achieved by a manufacturing method of the presentinvention which can be applied to both species of the semiconductorlasers of the patterned-substrate type, a first species having a lasingregion on a planar portion of the patterned-substrate and a secondspecies having a lasing region on a sloped portion of thepatterned-substrate, and the method characterized by the followingfeatures.

One method of the invention is applied to the first species ofsemiconductor lasers, the method including a step of growingcurrent-blocking layers on a planar portion of a substrate and on bothside surfaces of a mesa-stripe on the substrate, thus thepatterned-substrate being formed, the patterned-substrate having thecurrent-blocking layers with a gentle sloped surface without depressionresulting in obtaining a planar structure of the heterostructure on thetop of the mesa-stripe without undulation.

Another method of the invention is also applied to the first species ofsemiconductor lases, the method including a step of growing lower and/orupper clad layers, wherein a thickness ratio A/B of the clad layerhaving a predetermined value, for example, within a range from 1 to 2can be obtained, where A is defined for a thickness at the slopedportion and B is defined for the planar portion of the grown layer.

Still another method of the present invention is applied to a secondspecies of semiconductor lasers of patterned-substrate type, wherein asloped portion of an active layer emits lasing light. In the similar wayas in the first species, a thickness ratio of a sloped portion to aplanar portion of a clad layer lying close to the active layer iscontrolled so as to have a predetermined value, thereby the lasingportion in the active layer being well controlled without fluctuation.

Other objects and advantages of the present invention will become moreapparent from the detailed description to follow taken in conjunctionwith the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross sectional view of a semiconductor laser comprisingan AlGaInP-GaInP-AlGaInP heterostructure of the prior art,

FIG. 2 shows a cross sectional view of another semiconductor laser ofthe patterned-substrate type comprising an AlGaInP-GaInP-AlGaInPheterostructure in order to explain the prior art manufacturing methodand problems thereof,

FIGS. 3(a)-3(d) through FIGS. 8(a)-8(c) are related with a first speciesof semiconductor lasers of the patterned-substrate type of the presentinvention, where;

FIGS. 3(a) through 3(d) show a cross sectional view of sequential stepsduring fabrication of a semiconductor laser in accordance with thepresent invention for a purpose of explaining the principle of amanufacturing method of the present invention,

FIGS. 4(a) and 4(b) show a cross sectional view of thepatterned-substrate alone, wherein current-blocking layers of FIG. 4(a)have a depression on its surface but current-blocking layers of FIG.4(b) formed by the present invention have a gently sloped surface withless inclination,

FIGS. 5(a) and 5(b) show a cross sectional view of a heterostructuregrown on the patterned-substrate, thereby undulation being formed inheterostructure of FIG. 5(a), and no undulation being observed in FIG.5(b),

FIGS. 6(a) and 6(b) show a cross sectional view of an enlarged lasingportion of the heterostructure, and FIG. 6(a) shows schematically thatsplit transverse lasing modes are liable to occur, however, FIG. 6(b)shows a single transverse mode is generated,

FIGS. 7(a) and 7(b) show measured data of ratio A/B, where FIG. 7(a) isobtained by changing growth temperatures with constant V/III ratio of100, and FIG. 7(b) is obtained by changing V/III ratio with constantgrowth temperature of 720° C., and

FIGS. 8(a) through 8(c) show cross sectional views at differentmanufacturing steps of a semiconductor laser according to an embodimentof the present invention,

FIG. 9 through FIG. 13 are related with the second species ofsemiconductor lasers of the patterned-substrate type of the presentinvention, where

FIG. 9 shows a schematic cross section of a semiconductor laser of thepatterned-substrate type in the second species,

FIGS. 10(a) and 10(b) show a cross sectional view of upper clad layer,active layer and patterned substrate, wherein thickness ratios of theclad layers are different in FIGS. 10(a) and 10(b), thereby causingcurrent injection modes to change,

FIG. 11 shows a geometrical analysis to determine a thickness ratio ofthe upper clad layer,

FIG. 12 shows measured data of thickness ratio R by changing a substratetemperature during growth, while V/III ratio is maintained at 180 and330, and

FIG. 13 shows a cross sectional view of a semiconductor laser inEmbodiment 2 in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principle of the manufacturing method in accordance with the presentinvention is first described on a semiconductor laser of thepatterned-substrate type classified in the first species. FIGS.3(a)-3(d) through FIGS. 8(a)-8(c) are related with the first species.

The patterned-substrate type laser of the present invention requires asubstrate 1 with a mesa-stripe 1a as shown in FIG. 3(a). In order toobtain the substrate 1 of this shape, an original flat substrate of GaAshaving a (100) surface is provided, and a SiO₂ layer is deposited on the(100) substrate surface by sputtering. The SiO₂ layer is selectivelypatterned by photolithography leaving a stripe pattern 2 of SiO₂ layer,thereby the stripe pattern being formed in the <110> direction of the(100) GaAs substrate.

It is known that surface and direction of a crystal, though they areexpressed by different indices, comprise the equivalent surfaces anddirections in crystallography, therefore, all equivalent surfaces anddirections are expressed by a representative surface index and directionin the description of the embodiment.

Using the stripe-shaped SiO₂ layer 2 as a mask, the substrate issubjected to a wet etching process, which results in forming thesubstrate 1 with the mesa-stripe 1a having a height of about 1.8 μm asshown in FIG. 3(b). There appears a (111)B surface on both sides of themesa-stripe 1a.

In the above step, a width 2a of the SiO₂ mask 2 is selected to be lessthan a bottom width 1c of the mesa-stripe 1a, and the mask 2 ispreferably to protrude outwardly beyond the top width of themesa-stripe. The outward protruding length is selected to be about 0.2μm.

In FIG. 3(c) , using the mask 2, current-blocking layer is grown onlevel surfaces of the substrate 1 and on both side surfaces of themesa-stripe 1a by a MOVPE method. Trimethyl-gallium (CH₃)₃ Ga(hereinafter abbreviated briefly as TMG) and arsine AsH₃ are used assource gases, and further a doping gas, for example, SeH₂ is introducedif a negative-type GaAs growth is required.

After the patterned-substrate comprising the substrate 1 with themesa-stripe 1a and the current-blocking layers 3 is produced in thisway, the SiO₂ mask layer 2 is removed by wet-etching using hydrofluoricacid HF, and a structure of FIG. 3(c) is obtained.

In FIG. 3(d) , a buffer layer 4 of GaAs, a lower clad layer 6 ofAlGaInp, an active layer 8 of GaInP, an upper clad layer 10 of AlGaInPand a contact layer 12 of GaAs are grown on the patterned-substrate.Only five layers are shown in FIG. 3(d) for a purpose of simplifying theexplanation. With regard to a polarity type of these layers, thesubstrate 1, the buffer layer 4 and the lower clad layer 6 are of afirst polarity type, and the upper clad layer 10 , the contact layer 12and the current blocking layer 3 are of a second polarity type oppositeto the first polarity type. Detailed composition and structure aredescribed in the subsequent description on the actual embodiment .

Generally, trimethyl-gallium TMG and triethyl-gallium (C₂ H₅)₃ Ga(hereinafter abbreviated briefly as TEG) are known as a source gas forgrowing a gallium compound semiconductor layer but, if the TEG is usedfor growing current-blocking layers 3, depression 40 as shown in FIG.4(a) are formed on a surface 3a of the grown current-blocking layer 3 ofGaAs. The grown surface 3a gradually slopes down from the top of themesa-stripe, the sloped surface being close to a (311)B surface andcontinues to a comparatively flat surface, thereby forming thedepression 40 therebetween. The depression thus formed becomes a maincause of splitting of the transverse lasing mode and deteriorates a farfield pattern of the lasing light when the heterostructure issubsequently formed thereon.

When the heterostructure is grown on the patterned-substrate of FIG.4(a), undulation 42 caused by the depression 40 is observed in theheterostructure, which is shown in FIG. 5(a). When the laser structureof this type is used in application, transverse mode splitting willoccur. FIG. 6(a) shows a partial cross section of the heterostructurealone of the lower clad layer 6, the active layer 8 and the upper cladlayer 10. The FIG. 6(a) schematically shows the transverse mode of thelasing light 15 are split into three patterns.

In accordance with the present invention, the TMG gas is intentionallyused for the growth of the GaAs current-blocking layer 3. In FIG. 4(b),slope of the grown surface 3a becomes more moderate, and the surface isa slightly curved surface, the surface inclination being close to a(411)B surface of the substrate 1 having a (100) surface. Sinceconspicuous depression disappears on the surface , the heterostructureformed on the thus formed patterned-substrate comprises flat layers onthe mesa-stripe without undulation. FIG. 5(b) shows schematically thethus formed heterostructure, and the transverse mode of the lasing lightis shown in FIG. 6(b) in which a single mode alone appears.

In order to achieve the objects of the present invention and realizeimproved characteristics of the semiconductor lasers, we found that, inthe semiconductor laser of the patterned-substrate type, it is animportant factor to select a thickness ratio of a specific semiconductorlayer at an optimum value, the ratio being defined for the slopedportion to the planar portion of the semiconductor layer.

First, the buffer layer 4 of FIG. 3(d) which is directly deposited onthe patterned-substrate should have a sufficiently high electricalresistance on the current blocking layer 3 but have a low electricalresistance on the mesa-stripe. Since a carrier depletion layer is formedat an interface between the buffer layer 4 and the current-blockinglayer 3. It is desirable that the buffer layer on the current blockinglayer 3 works as a high resistance layer caused by the carrierdepletion, but the buffer layer 4 has not too much resistance on themesa-stripe. As the compromise, we found an impurity concentration of1×10¹⁷ cm⁻³ and a thickness of 50 nm on the mesa-stripe is preferable asthe buffer layer 4. A thickness ratio C/D of the buffer layer 4 shown inFIG. 3(d) is selected to be less than 1, where C denotes a thickness atthe sloped portion of the buffer layer 4 and D denotes a thicknessthereof above the mesa-stripe. The ratio C/D of less than 1 means thebuffer layer on the mesa-stripe has a required thickness but a thicknesson the current-blocking layer 3 preferably decreases as much aspossible.

Second , a thickness ratio A/B is defined for the lower clad layer 6,the upper clad layer 10, and the contact layer 12, where A denotes athickness at the sloped portion of each layer and B denotes a thicknessthereof above the mesa-stripe in the similar way as the ratio C/D. It isdesirable that the grown layers have an appropriate value of ratio A/B.The symbol A/B is used for the growth of AlGaInP clad layers 6, 10 andGaAs contact layer 12.

Our experiments showed it is preferable that the ratios A/B for thelower and upper clad layers 6, 10 are selected in a range between 1 and2. This results in increasing an electrical resistance in the thicknessdirection at the sloped portions of each layer, thereby preventing anelectrical resistance above the mesa-stripe from increasing. As theresult, laser current flow is more concentrated in the lasing portionabove the mesa-stripe.

In growing an AlGaInP-GaInP-AlGaInP heterostructure on thepatterned-substrate, the ratio A/B of the grown layers varies dependingon a source gas ratio α (V/III: a flow ratio of a group V source gas tosum of group III source gases) and a substrate temperature Tg duringgrowth. An example of measured data is shown in FIGS. 7(a) and 7(b).FIG. 7(a) shows measured data on a relation between A/B and growthtemperature, thereby α being fixed at 100. FIG. 7(b) shows measured dataon a relation between A/B and α, thereby the growth temperature beingfixed at 720° C. The both data are obtained under the constantconditions of Pg=50 Torr, E=800 μm/mol and Ft=8 SLM (standard liter perminute), where Pg denotes a gas pressure in the growth chamber, Edenotes a growth efficiency (thickness of grown layer per unit mol ofgroup III source gas supplied), and Ft denotes total gas flow amount. Aphosphine gas is used as the group V source gas, and TEG,trimethyl-indium (abbreviated briefly as TMI) and trimethyl-aluminum(abbreviated briefly as TMA) gases are used as the group III sourcegases.

From the measured data, the following relation is obtained,

    A/B=0.0039α-0.035Tg+26.05,                           (1)

where Tg denotes growth temperature in ° C.

In order to satisfy the condition of 1<A/B<2, α and Tg are required tosatisfy the following relation,

    -25.05<0.0039α-0.035Tg<-24.05                        (2)

If the gas pressure Pg (Torr), the growth efficiency E (μm/mol), and/orthe total gas flow Ft (SLM) are changed, α value applied to the aboverelation (2) is to be modified by inserting the following α value intoequation (2),

    α=128 ×α'×Pg/(E×Ft),         (3)

where α' denotes V/III ratio which is necessary in the growth apparatusunder modified growth conditions.

The above relations are obtained for, growing semiconductor layers ofAlGaInP and GaInP, in which a phosphine (PH₃) gas is used as a group Vsource gas. In case of growing a GaAs layer, where arsine (AsH₃) gas isrequired as a source gas, the above relations can not be applied.

In growing a GaAs semiconductor layer, the C/D ratio for the bufferlayer 4 and the A/B ratio for the contact layer 12 are more dominated bythe kind of group III source gas rather than the V/III ratio. In growingthe GaAs buffer layer 4, the C/D ratio of less than 1 is preferable aspreviously described. This condition can be achieved by using a TEG gasas the group III source gas. In growing the GaAs contact layer 12 ofFIG. 3(d), on the other hand, the use of a TMG gas is desirable. In thiscase, the A/B ratio of greater than 2 can be obtained, namely, acomparatively flat grown surface of the contact layer can be obtained.After forming an upper electrode on the contact layer, the thus formedflat surface alleviates strain caused by a subsequent bonding processand contributes to improve reliability of the lasers.

Herein it is noted again that, in the semiconductor laser of thepatterned-substrate type, it is an important factor to select athickness ratio of a specific semiconductor layer at an optimum value,the ratio being defined for the sloped portion of grown layer layer tothe planar portion thereof. This principle can be applied to the secondspecies of semiconductor lasers other than those in the first species.

FIG. 9 shows a schematic cross section of a semiconductor laser withpatterned-substrate in the second species. In FIG. 9 through FIG. 13,the same reference numerals designate the same or like parts used in thefirst species, therefore, detailed descriptions thereof are omitted.Reference numerals 10a, 10b, 10c denote portions of an upper clad layer10. The layer portion 10b has a function of a current blocking layer.

In the semiconductor laser in the second species shown in FIG. 9, alaser current flows to an active layer 8 through the sloped layerportion 10c. Although the entire sloped portion 8c or the active layer 8can emit lasing light, it is necessary that injection laser currentshould be injected uniformly onto the entire surface of the slopedportion 8c.

FIG. 10(a) is the similar cross section as FIG. 9 with a purpose ofexplaining flow condition of the injection current. Point C is assumedas the middle point of line DG. Line CE (in an actual structure, CE is aplane vertical to the drawing sheet, however, the term "line CE" is usedfor simplicity) divides the layer portion 10c vertically into two, andthe injected current flows on both sides of line CE and centered to theline CE because the direction CE indicates the shortest path to theactive layer 8.

Theoretically, if the point E (length CE is the shortest distance frompoint C to the active layer) is at the middle point of the slopedportion of the active layer 8, the center of the lasing light should beat the middle point of the active layer. In actual cases, it is foundvery difficult to keep the lasing center at the middle of the slopedportion of the active layer. The lasing center frequently moves in theactive layer.

In order to solve the above problem, a thickness ratio of the slopedlayer portion to the planar layer portion of the upper clad layer 10 isdetermined so as to position the point E on the lower sloped portion 8cof the active layer. This is illustrated in FIG. 10(b). When thicknesstp1 of the sloped portion of the upper clad layer and thickness tp2 ofthe planar portion thereof are determined as shown in FIG. 10(b), inother words, thickness tp1 is larger than thickness tp2, then a verticalline from the point C (C is the middle point of the upper surface of thesloped portion 10c) crosses the active layer 8c at point E that ispositioned on the lower half of the active layer 8c. It is founddesirable that the point E is positioned on the lower side of the slopedportion of the active layer as described above. In this case, the lasingcenter is restricted in the limited area of the lower portion of theactive layer 8c and fluctuation of the lasing center is remarkablyreduced.

In order to position the lasing center on the lower side of the slopedportion 8c of the active layer, the specific geometrical conditionshould be satisfied.

This condition may be found using FIG. 11, in which thickness of thesloped portion 10c is different from that of the planar portion 10bthereof. In FIG. 11, it is assumed that points C and A are the middlepoints of lines DG and BF respective; thickness of the sloped portion oflayer 10c is denoted as tp1; thickness of the planar portion of layer10b is denoted as tp2; angle between lines BD and RH (angle betweengrowth direction of the sloped layer and planar surface) is θ;inclination angle of the sloped layer is φ; CE is vertical to BF; andwidth of the sloped portion is W; then we have:

    R=sin φ [(cos θ/sin θ)+(cos φ/sin φ)],(4)

where R=tp1/tp2.

Condition for making point E coincident with point B is given by tanω=2tp1/W, and

condition for making point E coincident with point A is given by ω=90°;

where ω=θ+φ.

Hence, the condition that point E exists between points A and B is givenby the following condition,

    tan.sup.-1 (2tp1/W)≦θ+φ≦90°.(5)

If θ and ω (or φ) are determined to satisfy the above condition (5),then necessary thickness ratio R is given by equation (4).

Next, utilizing the MOVPE method, growth condition which satisfies thethus obtained thickness ratio R (=tp1/tp2) is given by the followingrelation,

    R=aTg.sup.2 +bTg+c+dα,                               (6)

where Tg denotes a growth temperature of the substrate in ° C., and αdenotes a source gas ratio V/III, and a, b, c, d are constants shownbelow

a=1.9999×10⁻⁴

b=-0.297985

c=111.505

d=1.6667×10⁻³.

The above condition is measured under the conditions of Pg=50 Torr,E=800 μm/mol and Ft=8 SLM and equation (6) is obtained from experimentaldata shown in FIG. 12, where the abscissa shows the substratetemperature and the ordinate shows thickness ratio R of the slopedportion (tp1) to the planar portion (tp2) of grown semiconductor layer.In FIG. 12, two curves are shown, one for α=180 and the other for α=330.Two curves are substantially parallel and may be expressed approximatelyby a quadratic curve. The equation (6) which expresses the relationbetween the thickness ratio R and the substrate temperature Tg can beobtained from these curves, thereby the V/III ratio α being used as aparameter.

If the gas pressure Pg (Torr), the growth efficiency E (μm/mol), and/orthe total gas flow Ft (SLM) are changed, α value applied to the aboverelation (6) is to be modified by inserting the following relation (7).This procedure is completely the same as that applied for modifying α incondition (2) using relation (3), which is described previously.

    α=128 ×α'×Pg/(E×Ft),         (7)

where α' denotes V/III ratio which is needed in an actual growthprocess.

The above relations are obtained for growing semiconductor layer using aphosphine (PH₃) gas as a group V source gas, therefore, the aboverelations can not be applied when an arsine (AsH₃) gas is used as asource gas.

Embodiment 1

FIGS. 8(a)-8(c) show schematic cross sections at different steps inmanufacturing a semiconductor laser in the first species according tothe present invention.

In FIG. 8(a), a p-type (100) GaAs substrate 1 (Zn-doped) is prepared. ASiO₂ layer 2 is deposited on the substrate 1 by sputtering andthereafter it is selectively patterned so as to form a stripe-shapedSiO₂ layer extending in the <110> direction. Using the stripe-shapedSiO₂ layer 2 as a mask, the substrate 1 is subjected to mesa-etching.This results in forming the GaAs substrate 1 having a mesa-stripe 1a,and there appear (111)B surfaces on both sides of the mesa-stripe. Theheight of the mesa-stripe 1a is controlled to be about 1.8 μm, and theSiO₂ mask layer 2 forms a protrusion of about 0.2 μm in length whichprojects outwardly from the edge of the top of the mesa-stripe.

Next as shown in FIG. 8(b), an n-type GaAs current blocking layer 3(Se-doped) having a thickness of about 1 μm is grown on the GaAssubstrate 1 by a MOVPE method, thereby TMG, AsH₃, and SeH₂ gases beingused as the source gases. A V/III ratio of 80 and a growth temperatureof 670° C. are used in this growth. The TMG gas is used in the growth asa Ga source gas with an object of forming a gently sloped flat surface,which is described previously. As the result, the grown current blockinglayer 3 has a substantially flat surface 3a which is to a (411)B surfaceof (100) GaAs. As substrate 1, the (411)B surface having an inclinationless than that of (311)B surface.

When the SiO₂ mask layer 2 is removed by hydrofluoric acid HF, thepatterned-substrate is obtained. In FIG. 8(c), a plurality ofsemiconductor layers 4 through 11 are grown thereon one after another bythe MOVPE method follows. Through all growth steps, conditions of gaspressure Pg=50 Torr, growth efficiency E=800 μm/mol, and total gas flowFt=8 SLM are maintained.

    ______________________________________                                                      Ref.  Doping    Thickness                                                     No.   Material  (μm)                                         ______________________________________                                        p-GaAs buffer layer                                                                           4       Zn        0.05                                        p-GaInP lower spike                                                                           5       Zn        0.1                                         reduction layer                                                               p-AlGaInP lower clad                                                                          6       Mg        0.5                                         layer                                                                         p-AlGaInP spacer layer                                                                        7       Mg        0.1                                         GaInP active layer                                                                            8       --        0.08                                        n-AlGaInP guide layer                                                                         9       Se        0.3                                         n-AlGaInP upper clad                                                                          10      Se        0.6                                         layer                                                                         n-GaInP upper spike                                                                           11      Se        0.1                                         reduction layer                                                               ______________________________________                                    

For growing these layers, TEG, TMA, TMI, AsH₃, PH₃, SeH₂, DMZn(dimethyl-zinc), (C₅ H₅)₂ Mg (cyclopentadienyl-magnesium) gases are usedas source gases and doping gas. The TMG gas is not used for the growthof these layers, however, the TMG gas may be mixed with the TEG gasduring the growth of the lower and upper clad layers 6, 10 in order tomodify a shape of the grown layer.

Thereafter, an n-type GaAs contact layer 12 having a thickness of 1 μmis grown on the above structure, thereby the TMG being used as a Gasource gas in order to obtain the A/B ratio of about 2 or greater than 2in order to form the flat surface as much as possible as previouslydescribed.

An upper electrode 13 of Au/AuGe is formed on the n-GaAs contact layer12 and a lower electrode 14 of Au/AuZn is formed on the bottom surfaceof the p-GaAs substrate 1, completing fabrication of the semiconductorlaser shown in FIG. 8(c) .

Embodiment 2

An embodiment 2 relates to the second species of semiconductor lasers ofthe patterned substrate type in accordance with the present invention.FIG. 13 shows a schematic cross section of this type of thesemiconductor laser. The semiconductor laser of FIG. 13 is manufacturedby the following method.

An n-type (100) GaAs substrate 1 (Si-doped) is provided. In the similarway as the embodiment 1, SiO₂ mask is formed in the <011> direction ofthe substrate. The substrate is subjected to an etching process forminga sloped portion of a (411)A surface. A depth of the etched portion isabout 1 μm.

After removing the mask, semiconductor layers are grown one by onesequentially on the patterned substrate 1. Throughout the growth,conditions of Pg=50 Torr, E=800 μm/mol and Ft=8 SLM are maintained usinga hydrogen gas as a carrier gas. Other main growth conditions areoutlined in the following table.

    __________________________________________________________________________                 Ref. Doping                                                                   No.  Material                                                                            Thickness (μm)                                                                     Tg, α                                   __________________________________________________________________________    n-GaAs first buffer                                                                         4   Se    1.0     Tg = 670° C.                           layer                           α = 50                                  n-GaInP second buffer                                                                       5   Se    0.1     Tg = 670° C.                           layer                           α = 400                                 n-AlGaInP lower clad                                                                        6   Se    P: 0.3  Tg = 710° C.                           layer                   S:0.6   α = 300                                 n-AlGaInP guide layer                                                                       7   Se    P: 0.2  Tg = 710° C.                                                   S:0.4   α = 300                                 GaInP active layer                                                                          8   --    P: 0.015                                                                              Tg = 710° C.                                                   S:0.03  α = 400                                 p-AlGaInP first upper                                                                      10a  Mg    P: 0.3  Tg = 710° C.                           clad layer              S:06    α = 300                                 n/p-AlGaInP second                                                                         10b/10c                                                                            Zn + Se                                                                             P: 0.2  Tg = 710° C.                           upper clad layer                                                                           (See remarks 2)                                                                          S: 0.4  α = 300                                 p-AlGaInP first spike                                                                      11a   Mg   0.04    Tg = 710° C.                           reduction layer                 α = 300                                 p-GaInP second spike                                                                       11b  Zn    0.1     Tg = 710° C.                           reduction layer                 α = 400                                 p-GaAs contact layer                                                                       12   Zn    5       Tg = 670° C.                           __________________________________________________________________________                                    α = 50                              

Remarks

(1): In the thickness column, "P:" denotes thickness of the planarportion, "S:" denotes thickness of the sloped portion.

(2): In growing the second upper clad layer, DMZn and SeH₂ gases aremixed and used as dopant gases. Since the planar portion and slopedportion of the upper clad layer have different surface indices andactivation efficiency of each dopant is different for two surfaces,therefore, this results in growing an n-type AlGaInP layer for theplanar portion 10b and a p-type AlGaInP layer for the sloped portion10c. The n-type AlGaInP planar portion 10b works as a current blockinglayer,

Source gases used for growing the above layers are substantially thesame as those used in Embodiment 1. TEG, TMA, TMI, AsH₃, PH₃ are used assource gases and SeH₂, DMZn, (C₅ H₅)₂ Mg are used as doping gases.

After the growth of the above layers 4 through 12, electrodes 13 and 14are respectively formed on the upper and lower sides of the grownstructure. After cleaving and coating on the end surfaces, semiconductorlaser chips are completed.

In the above Embodiment 2, the upper clad layer 10b, 10c has a functionof controlling the lasing current, however, this function is not limitedto the upper clad layer 10b, 10c, but the same function can be obtainedwith regard to the lower clad layer 6 instead of the upper clad layer.

The present invention may be embodied in other specific forms. Thepresently disclosed embodiment is, therefore, to be considered in allrespects as illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, rather than the foregoingdescription, and all changes which come within the meaning and range ofequivalence of the claims are, therefore, to be embraced therein.

What is claimed is:
 1. A manufacturing method of a semiconductor lasercomprising a heterostructure grown on a patterned-substrate, theheterostructure comprising at least a lower clad layer, an active layerand an upper clad layer, and the patterned-substrate comprising asubstrate with a mesa-stripe and current blocking layers burying bothsides of the mesa-stripe, said method comprising the following steps ofmanufacturing the patterned-substrate:providing a GaAs substrate havinga (100) surface, forming a selectively patterned mask layer having astripe shape on said substrate, the stripe being formed in the <110>direction of the substrate, etching selectively said substrate, forminga mesa-stripe having (111)B surfaces on both sides of the mesa-stripe,growing the current blocking layer of GaAs by a MOVPE method on saidsubstrate, wherein a trimethyl-gallium (TMG) is used as a gallium sourcegas, thereby the grown layer having a sloped surface on both sides ofthe mesa-stripe, the surface having an inclination smaller than that of(311)B surface of the substrate, and removing the mask layer.
 2. Amanufacturing method of a semiconductor laser as recited in claim 1,wherein said GaAs substrate is of a first polarity type and said grownGaAs current-blocking layer is of a second polarity type opposite to thefirst polarity.
 3. A manufacturing method of a semiconductor laser asrecited in claim 1, wherein said lower clad layer is of AlGaInP having afirst polarity type and said upper clad layer is of AlGaInP having asecond polarity type opposite to the first polarity and said activelayer is of non-doped GaInP.
 4. A manufacturing method of asemiconductor laser as recited in claim 1, wherein said grown surface ofGaAs current-blocking layer has an inclination close to that of a (411)Bsurface of the substrate.
 5. A manufacturing method of a semiconductorlaser as recited in claim 2, wherein a buffer layer is formed on saidpatterned-substrate, said method further comprises the steps of:growinga GaAs buffer layer of the first polarity type on saidpatterned-substrate by a MOVPE method, wherein a triethyl-gallium (TEG)is used as a gallium source gas, thereby a thickness of the buffer layeron the current-blocking layer is less than that on the mesa-stripe.
 6. Amanufacturing method of a semiconductor laser as recited in claim 3,wherein said AlGaInP lower clad layer and said AlGaInP upper clad layerare respectively grown by a MOVPE method, thereby triethyl-gallium(TEG), trimethyl-aluminum (TMA), and trimethyl-indium (TMI) gases beingused as group III source gases so as to obtain a thickness ratio A/Bwithin a range between 1 and 2, where A denotes a thickness of the grownlayer above the sloped surface of the current-blocking layer and Bdenotes a thickness of the grown layer above the mesa-stripe.
 7. Amanufacturing method of a semiconductor laser as recited in claim 6,wherein the growth of said lower and upper clad layers by the MOVPEmethod satisfies the following relation of

    -25.05<0.0039α-0.035Tg<-24.05,

where α denotes a gas flow ratio V/III (group V source gas flow to totalgas flow of group III gases) and Tg denotes a growth temperature in °C.8. A manufacturing method of a semiconductor laser as recited in claim1, said method further comprises a growing step of a GaAs contact layerafter growing said heterostructure by a MOVPE method, whereintrimethyl-gallium (TMG) is used as a group III source gas, thereby athickness ratio A/B of about 2 or greater than 2 being obtained, where Adenotes a thickness of the grown contact layer above the sloped surfaceof the current-blocking layer and B denotes a thickness of the growncontact layer above the mesa-stripe.
 9. A manufacturing method of asemiconductor laser of a patterned-substrate type, thepatterned-substrate having first and second planar portions and a slopedportion between the first planar portion and the second planar portion,said method comprising the steps of growing an active layer and a cladlayer of AlGaInP, the growing step of said clad layer is characterizedin that:said clad layer has thicknesses tp1 and tp2, each being definedfor a current channel region and a current blocking region correspondingto said sloped portion and planar portion respectively, wherein athickness ration R (=tp1/tp2) satisfies the following relation;

    R=sin φ [(cos θ/sin θ)+(cos φ/sin φ)],

whereby θ and φ are selected within a range specified by the followingcondition;

    tan.sup.-1 (2tp1/W)≦θ+φ≦90°;

where φ is defined for an inclination angle of a sloped portion of cladlayer (said current channel region), and θ is defined for an anglebetween growth direction of said clad layer and a planar surface, and Wis defined for a width of said sloped portion of said clad layer, andgrowth condition is determined so as to satisfy the following equation,

    R=aTg.sup.2 +bTg+c+dα,

where Tg denotes a growth temperature of the patterned-substrate in °C.,and α denotes a source gas ratio V/III for growing the AlGaInP cladlayer, and a, b, c, d are constants as follows, a=1.9999×10⁻⁴b=-0.297985 c=111.505 d=1.6667×10⁻³.
 10. A manufacturing method of asemiconductor laser as recited in claims 7 and 9, wherein said α ismodified to α' when growth conditions are changed by substituting thefollowing equation;

    α=128 ×α'×Pg/(E×Ft),

where Pg denotes a gas pressure in Torr, E denotes a growth efficiencyin μm/mol, and Ft denotes a total gas flow Ft In SLM, whereby the α'denotes a V/III ratio to be applied for the growth.
 11. A manufacturingmethod of a semiconductor laser as recited in claim 9, wherein ingrowing said clad layer, dopant gas comprising p-type and n-type dopantgases is mixed in the source gases, thereby an n-type AlGaInP planarlayer being grown on said planar portion and a p-type AlGaInP slopedlayer being grown on said sloped portion due to a difference inactivation efficiency of the dopant gases.